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"It will use that information to predict the number of calories that you'll burn during your exercise," he said.

But this doesn't necessarily mean the prediction is always correct.

What did the researchers find?

The study, which featured on the BBC X-Ray programme, measured the amount of oxygen used by participants while exercising on a treadmill - and compared the results to those recorded by the fitness trackers.

Researchers found that there was "considerable variation" in the number of calories the devices said participants had burned, but explained that there's a "fairly consistent pattern".

Dr Thatcher said some of the devices had a tendency to over-predict, while others estimated below the actual number of calories burned.

The study found that whilst running some fitness trackers miscalculated by up to 40% (Image: Shared Content Unit)

For example, when participants ran on the treadmill, the Fitbit product was within about 5% of the actual amount burned.

The Letsfit and Letscom instead under-predicted the number of calories burned by around 30-40%.

The study however found that data recorded while walking at speed was even more inaccurate.

Aberystwyth University researchers found that Letsfit and Letscom were within around 15-20% of the actual calories burned - whilst Fitbit over-predicted by about 50%.

Dr Thatcher noted that the miscalculation of devices may be linked to the fact that we all burn calories when we're not doing exercise, known as your resting metabolic rate.

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Does it really matter if fitness trackers miscalculate the number of calories burned?

Discussing the findings on the Welsh consumer programme, Dr Thatcher said it depends on how you use the devices.

He explained that users shouldn't be too concerned if the devices are being used as a motivational tool to track activity over a period of time.

"However if you want the exact number of calories you are burning during an exercise session then it doesn't matter which device you have, you have to interpret the data with some caution," he said.

Dr Thatcher concluded that price doesn't always equate to accuracy, as "the more expensive devices have got as much variation as the cheaper devices".

Popular fitness trackers could be overpredicting the amount of calories burned while walking by 50% (Image: Getty Images/iStockphoto)

In response to the finding, Fitbit said they were "confident" in the performance of their products, which are based on "extensive scientific research".

The other two companies explained that the accuracy of devices depends on inputted data by users, and stressed that their products are not "scientific or medical devices", which provide only estimates.

Episode 10 of the current series of X-Ray is now available to watch on BBC iPlayer.